just wondering if it can be done

Simple backuping of the "spr" and "mpr" folders will preserve your replays in case of LFS reinstallation, disk format and such. As long as the replays were recorded by the same physics engine, this will work. There is no backward compatibility in LFS so any change to the physics engine will make the replays obsolete and useless... But you can always keep a backup of old version of LFS if you have some really astonishing replays that are worth it

Replays are normal files. Any suitable, normal backup technique will work. In the case of hosted servers, contact whoever is in charge.